In mammalian cells, cholesterol is an essential component for membranogenesis and for the synthesis of sterols and nonsterols that are critical for normal cellular functions. Excess cholesterol, however, not only is lethal to cells but also creates a major problem in atherolsclerosis for its deposit in arteries. To maintain cholesterol homeostasis, cells, in particular liver cells, adopt three major ways to regulate cholesterol levels: 1) uptake of dietary cholesterol via LDL receptor; 2) endogenous cholesterol biosynthesis and 3) metabolic conversion of cholesterol to bile acids. The key molecule that coordinates these processes is cholesterol itself, serving as a feedback signal. When the intracellular cholesterol level increases either through cholesterol uptake or biosynthesis, the transcription of genes including LDL receptor and the key cholesterol biosynthesis enzymes such as HMG-CoA synthase and HMG-CoA reductase is repressed. These feedback processes are mediated by a novel family of transcription factors called sterol regulatory element binding proteins (SREBPs). SREBPs contain an N-terminal transcription factor domain, two hydrophobic transmembrane domains and a C-terminal regulatory domain. When the intracellular cholesterol level is low, a two-step proteolytic cascade occurs which releases the N-terminal transcription factor domain of SREBPs from the endoplasmic reticulum, moving to the nucleus where activation of the SRE-containing genes occurs.
While the SREBP pathway is responsible for regulation of genes involved in cholesterol uptake and cholesterol biosynthesis such as LDL receptor and HMG-CoA synthase, the molecular basis of cholesterol catabolism is largely unknown. The major catabolic pathway for cholesterol removal is the production of bile acids that occurs exclusively in the liver. Cholesterol 7.alpha.-hydroxylase is the first and rate-limiting enzyme in the pathway. The cholesterol 7.alpha.-hydroxylase gene, also known as CYP7, belongs to the cytochrome P-450 family that contains many microsomal enzymes involved in liver metabolism. It has been shown that the expression of the CYP7 gene is tightly regulated: it is expressed exclusively in liver; its expression can be induced by dietary cholesterol and suppressed by bile acids. It has been shown that cholesterol catabolism plays a central role in cholesterol homeostasis. Treatment of laboratory animals with cholestid or cholestyramine, two bile acid-binding resins, decreases serum cholesterol levels. Moreover, overexpression of the CYP7 gene in hamsters reduces total and LDL cholesterol levels. Thus, cholesterol 7.alpha.-hydroxylase is a potential therapeutic target for cholesterol lowering drugs and understanding the mechanisms by which expression of the CYP7 gene is regulated is of particular importance.
To study the molecular mechanisms of hepatic-specific expression of the human CYP7 gene, we used HepG2 cells as a model system since this cell line is one of the most studied hepatic cell lines and has been shown to be an appropriate cell line through studies of a number of hepatic-specific genes including the CYP7 gene. We started with DNase I hypersensitivity mapping of the human CYP7 promoter and identified a hepatic-specific element in the promoter. Consequently, we cloned the gene encoding the promoter-binding protein and identified it as a human ortholog of the nuclear orphan receptor Ftz-F1 family.